docs: def labels(self, leaves=True, internal=True):
          '''Generator over the (non-``None``) ``Node`` labels of this ``Tree``
  
          Args:
              ``leaves`` (``bool``): ``True`` to include leaves, otherwise ``False``
  
              ``internal`` (``bool``): ``True`` to include internal nodes, otherwise ``False``
          '''
          if not isinstance(leaves, bool):
              raise TypeError("leaves must be a bool")
          if not isinstance(internal, bool):
              raise TypeError("internal must be a bool")
          for node in self.traverse_preorder():
              if node.label is not None and ((leaves and node.is_leaf()) or (internal and not node.is_leaf())):
                  yield node.label